The invention relates to a hydraulic vehicle brake system, and particularly to an improved brake system of the type having an external-force service brake system and a muscle-powered secondary brake system.
A vehicle brake system of the type with which this invention is concerned is known from International Patent Disclosure WO 98/31576. For external-force braking, this known vehicle brake system has a hydraulic pump as well as a high-pressure hydraulic reservoir, connected to a pressure side of the hydraulic pump, with which reservoir, wheel brake cylinders can be subjected to pressurized brake fluid via pressure buildup valves for the sake of the external-force braking. To lower the pressure in the wheel brake cylinders, these wheel brake cylinders are connected via pressure reduction valves to a low-pressure hydraulic reservoir that is disposed on an intake side of the hydraulic pump. With the pressure buildup valves and pressure reduction valves, antilock and slip control are possible in a manner known per se by modulation of the brake pressure in the wheel brake cylinders.
For secondary braking, the known vehicle brake system has a master cylinder, with which the wheel brake cylinders can be actuated directly. The secondary braking is provided in the event of a failure of the external-force service brake system. In the function of the external-force service brake system, during braking the master cylinder is disconnected from the wheel brake cylinders by disconnecting valves; that is, in the event of the external-force braking, the pressure buildup in the wheel brake cylinders is not accomplished with the master cylinder but rather solely with the hydraulic pump. In the case of the external-force braking, the master cylinder acts as a set-point transducer for a pressure to be established in the wheel brake cylinder, or in other words for the braking force.
During an external-force braking with disconnecting valves closed, to obtain an actuating travel at the master cylinder, the known vehicle brake system has a so-called active pedal travel simulator. The pedal travel simulator includes not only the master cylinder, the low-pressure hydraulic reservoir and high-pressure hydraulic reservoir, but also two pedal travel valves, which are disposed one between the low-pressure hydraulic reservoir and the master cylinder and one between the high-pressure hydraulic reservoir and the master cylinder. Upon actuation of the master cylinder, brake fluid can be positively displaced out of the master cylinder into the low-pressure hydraulic reservoir by opening the one pedal travel valve, in order to obtain an actuating travel at the master cylinder. A requisite actuating force is controlled with the pedal travel valve, as a function of an actuating travel of the master cylinder, in such a way that a conventional or desired pedal travel/pedal force characteristic is attained. For restoration of the master cylinder, the other pedal travel valve is opened, and the master cylinder is restored by brake fluid from the high-pressure hydraulic reservoir. The restoring force is controlled by this pedal travel valve.
The known vehicle brake system just described has the disadvantage of having a large number of valves. Another disadvantage is that brake circuit disconnection is not possible.
To embody an active pedal travel simulator, the vehicle brake system according to the invention has a disconnecting valve by way of which the master cylinder communicates with the low-pressure hydraulic reservoir, and a return valve by way of which the master cylinder communicates with the high-pressure hydraulic reservoir. This makes for economy by dispensing with pedal travel valves. The external-force braking is done, as in the known vehicle brake system, by subjecting the wheel brake cylinders to pressure from the high-pressure hydraulic reservoir, which is filled by use of the hydraulic pump. The master cylinder serves as a set-point brake force transducer. By means of the disconnecting valves, upon actuation of the master cylinder, brake fluid is let out of the master cylinder into the low-pressure hydraulic reservoir, and the quantity of brake fluid released can be controlled with the disconnecting valve, so that a desired actuating travel/actuating force characteristic at the master cylinder can be established, which imparts a familiar or desired brake actuation feeling to the driver in the external-force braking. A restoration of the master cylinder is accomplished by opening the return valve with brake fluid from the high-pressure hydraulic reservoir; the restoring force of the master cylinder can be controlled with the return valve. Another advantage of the vehicle brake system of the invention is that it can be embodied as a multi-circuit brake system, with mutually independent brake circuits, thus enhancing the safety of the vehicle brake system.